Enable C assertions in unit tests

Author: lpsinger

Diff for: tox.ini

@@ -10,8 +10,11 @@ requires =
 isolated_build = true
 
 [testenv]
-# Suppress display of matplotlib plots generated during docs build
 setenv =
+    # Enable assertions
+    CFLAGS=-UNDEBUG
+
+    # Suppress display of matplotlib plots generated during docs build
     MPLBACKEND=agg
 
     devdeps: PIP_EXTRA_INDEX_URL = https://pypi.anaconda.org/astropy/simple https://pypi.anaconda.org/liberfa/simple https://pypi.anaconda.org/scientific-python-nightly-wheels/simple